1. Field of the Invention
The present invention relates to a laser, more particularly relates to a laser having an external ground fault protection device.
2. Description of the Related Art
In the past, lasers have been broadly used in the fields of cutting, welding, surface treatment, and other processing. In particular, CO2 gas lasers are high in efficiency and give a high output, so are being used in the field of laser processing.
A CO2 gas laser uses a blower to make the laser gas containing the CO2 gas circulate while flowing at a high speed between the discharge electrodes so as to raise the output of the laser. Further, the motor of this blower is arranged in a gas circulation path through which the laser gas circulates. This motor is used in a state where its withstand voltage drops compared with under atmospheric pressure since it operates under a reduced pressure. Further, this type of laser operates by receiving the supply of power from a 3-phase AC power line.
Among the factories where such CO2 gas lasers operate, many factories were built considerably long years ago. Many factories may even be found not equipped with protection devices against ground faults of electrical equipment. If a ground fault occurs in electrical equipment in such a factory, the CO2 gas laser electrically connected to the same wiring system as this electrical equipment will experience an abnormal rise in the voltage between the phase of the power supply where the ground fault occurs and the ground and in some cases the motor of the blower will experience insulation breakdown.
To prevent such an accident due to an external ground fault, there are devices using a zero phase current detection current transformer (CT) or zero phase voltage detection voltage transformer (PT) etc. to detect an external ground fault. However, these are bulky in themselves and are also high in price. These factors become obstacles in newly introducing devices for prevention of external ground faults.
On the other hand, ground fault detection devices of electrical systems where motors are connected as loads have been proposed.
For example, Japanese Patent Publication (A) No. 8-322140 discloses a ground fault detection device suppressing saturation of a current transformer by the DC component of a rush current at the time of motor startup to enable prevention of unnecessary operation of a motor protection electric relay and proposes the means of using a coil to detect a ground fault.
However, what the ground fault detection device described in Japanese Patent Publication (A) No. 8-322140 detects is a ground fault inside the unit. It was difficult to detect an external ground fault.